1. Field of The Invention
This invention generally relates to a module for optical communication, which supplies an optical signal to an optical fiber or receives and detects an optical signal transmitted through an optical fiber. More particularly, this invention relates to a module for optical communication, which can prevent the reflection of light on an end surface of an optical fiber and can position an end surface of an optical fiber at an image point where an image is formed from light having passed through a collective or converging lens, with high precision.
2. Description of The Prior Art
Generally, a module for optical communication requires high precision connection between an optical fiber and a light emitting or receiving element.
FIG. 8 is a partially sectional view of a conventional module for optical communication, which is equipped with an optical-fiber ferrule. The illustrated module is of the same type as disclosed in the description of &lt;Embodiment 2&gt; in the upper-left column on page 3 and FIG. 3 of the Japanese Patent Public Disclosure Official Gazette (Kokai Koho) No. 2-50110/1990.
Reference numeral 1 designates a housing (or receptacle) of the conventional module for optical communication. A holding member 2 is screwed to the bottom portion of the housing 1. A recess portion 2a is formed in an upper internal circumferential portion of the holding member 2. Further, the outer circumferential portion of a transparent plate 3 is sandwiched between this recess portion 2a and the housing 1. The transparent plate 3 is made of a material which is equal in refractive index to the material such as quartz of the core of the optical fiber. Further, the transparent plate 3 has a thickness of 0.5 mm or so. Moreover, an anti-reflection coating is formed on the bottom surface of the transparent plate 3 as viewed in this figure.
The transparent plate 3 is produced in the shape of a flat plate while being in a free state. When the housing is equipped with a ferrule 4, the ferrule 4 is pushed against the transparent plate 3 by elastic force or resilience F. Further, this resilience F deforms the transparent plate 3, so that the transparent plate 3 is concaved as illustrated in FIG. 8. Thereby, an end surface 5a of an optical fiber 5 held by the ferrule 4 and the surrounding end surface of the ferrule 4 can adhere closely to the transparent plate 3 securely and elastically.
In this conventional module for optical communication, a light emitting element realized by employing a semiconductor laser and a converging lens for focusing laser light are provided. An image is formed on the end surface 5a of the optical fiber 5 from laser light B converged by the converging lens. Thus an optical signal is input to the optical fiber 5. In the case of this conventional module, the anti-reflection coating formed on the bottom surface of the transparent plate 3 as viewed in FIG. 3 prevents the laser light B from being reflected and returned to the semiconductor laser. Further, the reflection of the laser light B on the end surface of the optical fiber 5 can be prevented due to the fact that the end surface of the optical fiber 5 adheres closely to the transparent plate 3 and thus there is no space therebetween.
Moreover, in the case of the aforementioned conventional module, when the ferrule 4 is inserted into the housing 1, the transparent plate 3 is pressed by the end surface of the ferrule 4, so that the shape of the transparent plate 3 is changed into that indicated by dashed curves in FIG. 8. Therefore, the position in a direction, in which the center axis of the end surface 5a of the optical fiber 5 extends, changes according to variation in the pressure or resilience of the ferrule 4. Thus the position of the end surface 5a thereof is unsettled.
If the elastic force F is high, the end surface 5a of the optical fiber 5 moves downwardly as viewed in this figure. In contrast, if the force F is low, the end surface 5a thereof moves upwardly. Therefore, it is difficult to make the position of the image point f, at which an image is formed from the laser light B, coincide with the position of the end surface 5a of the optical fiber 5. Further, the amount of the difference in position between the image point and the end surface 5a changes according to variation in the resilience.
Moreover, as stated above, the transparent plate 3 has a large thickness of 0.5 mm or so. Thus, when the transparent plate 3 is deformed by the resilience F, the thickness of this plate varies owing to the deflection thereof. This also causes a difference in position between the image point f and the end surface 5a.
Owing to these causes, the difference in position between the end surface 5a of the optical fiber 5 and the image point f, at which an image is formed from the laser light B, is brought about when the ferrule 4 is inserted into the housing 1. This results in reduction in precision with which an optical signal is input to the optical fiber 5.
Furthermore, an addition of the holding member 2 having the recess portion 2a to the module is also due to the large thickness of the transparent plate 3. Thus, components such as screws for fastening the holding member 2 to the housing 1 become necessary. Consequently, the number of composing elements of the module for optical communication increases and the structure of the module becomes complex.
The present invention is accomplished to solve the problems concerning the conventional module.
Accordingly, an object of the present invention is to provide a module for optical communication, which can achieve the prevention of the reflection of light on an end surface of an optical fiber with simple structure and can regulate the position of an end surface of an optical fiber relative to a converging lens with high precision.